I've never seen a wire stretcher that was worth it's weight that did not have gator teeth slides on the bottom of it to hook the legs to or the lower half of a trapper style skinned hide.
They slide up and down, one on each side. Slide it up, hook it to the hind end of the hide or the leg, and pull it down, The tension of the hide wanting to pull it back up cocks the assembly in a way that the tension holds it where you leave it.